The Uitmarkt (website in Dutch only) is the yearly presentation of the new cultural season. The 3 day event consists mainly of some 400(small) free performances that give an impression of what a theatre or group will bring in the new season.
The Uitmarkt attracts a few hundred thousand visitors each year.

The online map gives you an idea of where the locations are, just East of Central Station.

The big opening performance is Friday evening, 8.50pm - 10pm, at the tip of the Java Island ('UPC podium' on the map).
Another highlight is the closing act by Cirque du Soleil, also on the UPC stage, Sunday August 26, 10.30pm to 11pm.

It can be difficult to see a particular performance, because of the large number of visitors, but the Uitmarkt is a nice opportunity to stroll around the new Eastern Harbour development along the IJ river behind the station.